1. Field of the Invention
The present invention relates to a charge pump attached to a center section in a hydrostatic transmission (hereinafter referred to as an "HST"), and more particularly to a charge pump whose support mechanism is simple in construction and inexpensive to produce.
2. Related Art
It is well-known that a charge pump for supplying operating oil to an HST may be mounted in close contact with one surface of a center section of the HST. The charge pump may be housed in a housing and driven by an input shaft of the HST which perforates through the center section. Such an arrangement disclosed in Japanese Utility Model Publication No. Hei 6-12276 which was filed by the same applicant as the present application. In such an arrangement, the pump casing which contains the charge pump is flexibly supported by a disc spring (rather than by bolts) disposed between the lower surface of the center section and the inner bottom surface of the housing. The charge pump is biased toward the horizontal lower surface of the center section so as to come into close contact therewith. This provides a charge pump with a self-charging relief function.
In such the conventional construction, a pin extends from the upper surface of the pump casing and is retained in the horizontal lower surface of the center section. As a result, the pump casing is not rotatable around the input shaft and the discharge port of the charge pump is maintained in proper communication with an operating oil supply port of the center section. Furthermore, the charge pump can move axially of the input shaft during the relief operation. When the charge pump is driven by the input shaft, a radial force or bending stress, generated at the pump casing is placed on the pin which acts as a shearing force on the pin, causing concern that the pin may be severed. Hence, a tough and highly rigid material should be used for manufacture of the pin. Therefore, the number of parts required in this arrangement is greater, leaving room for improvement in this known design.
The above-described spring for creating charge relief pressure uses a disc-like spring of corrugated annular plate-type. Such a spring is relatively larger in spring constant, creating a problem when variations exist during assembly in that the relief setting pressure may differ from one device to the next.
Further, the horizontal lower surface of the center section, with which the charge pump comes into close contact, needs to be machined with high accuracy in order to reduce the rotational and sliding resistance of the charge pump. This raises the processing cost. Moreover, discharge oil from the charge pump, when the accuracy of the machining processing is poor, leaks from the sliding surface of the charge pump resulting in a reduction in the efficiency of the device.